In atomic spectrometric methods, the sample, usually in the form of solution, is aspirated by means of a pneumatic nebulizer into a spray chamber from which it passes in the form of fine aerosol into the atomizer (flame AAS) or into the emission (mass) source (ICP-OES, ICP-MS). First, desolvation takes place, i.e. solvent evaporates resulting in so called “dry aerosol” of small solid particles (microparticles, nanoparticles) e.g. salt crystals. These particles enter the atomizer and then undergo several thermal rearrangement reactions (melting, volatilization, dissociation, atomization, excitation, ionisation). The precise reactions that occur will depend on the atomizer temperature and reaction partners (flame gases, ions) generated in the atomizer (emission source) or from the sample matrix. For example, hydrated chlorides, formed by many elements from hydrochloric acid solutions, can form oxides under the elimination of hydrogen chloride. Oxides can also be formed from carbonates, nitrates, etc.
The efficiency of solution transport (i.e. the analyte transport) into the atomizer (flame, plasma) depends on the rate of aspiration and the efficiency of nebulization. Under constant experimental conditions these factors depend on physical characteristics, such as viscosity, surface tension, vapour pressure, and density of the solution. Since most organic solvents have a lower viscosity and a lower specific mass than water they are more easily aspirated. The surface tension, which is also often substantially lower in organic solvent or oil based matrices, leads to finer nebulization (smaller droplets, smaller particles). This in turn ensures that considerably more sample reaches the atomizer (plasma) per unit time in organic or oil based samples. Moreover, many metal atoms are presumed to be more easily released from organic compounds than from various inorganic compounds, since organic molecules are thermally less stable than inorganic molecules. The higher the proportion of analyte ions aspirated and/or released for detection the higher the ratio of detector signal to analyte concentration. Therefore high quality accurate analysis is only achieved when calibration standard matrix closely matches that of the sample to be analysed.
To calibrate analytical spectroscopic methods for the analysis of water based test samples is relatively easy. Aqueous calibration standard solutions are prepared from well defined and pure starting raw materials (metals, inorganic salts) by dissolving them in water or in mineral acids. The analyte is completely dissolved and is present in water solution usually in an ionic form. When the test sample matrix components are added into the calibration standards, sufficient physical and chemical matching of calibration standards and sample solutions is usually achieved. Aqueous calibration standard solutions are commercially available.
Serious problems arise when aqueous calibration standard solutions are used when attempting to calibrate some so called solid sampling techniques (laser ablation, electrothermal vaporization, slurry nebulization etc.). In these techniques sample analyte enters the atomizer (plasma) in the form of solid particles or is already vaporized. In either case there is a very poor match between aspiration and release mechanisms of the analyte in the aqueous calibration standard and those of the analyte in the sample being tested. However, it has been confirmed experimentally that if the slurry particles are sufficiently small (less than 2 microns), both the analyte transport efficiency of the slurry particle through the sample introduction system and the atomization efficiency of the particle in the plasma are practically identical with those of aqueous calibration solution.
When organic solvents or other organic fluids (e.g. oils) are used for the sample preparation (dissolution), the situation regarding the calibration of atomic spectroscopic techniques is rather complicated. Aqueous calibration standards cannot be evidently used even though there have been many attempts to add aqueous standards into water miscible organic solvents, to prepare water-oil emulsion based standards etc. Standards based on the dissolution of organometallic compounds suffer from the lack of suitable, i.e., well defined, stable, soluble, sufficiently pure, organometallic compounds commercially available. Thus, as starting raw materials synthesized organometallic compounds (metal cyclohexanebutyrates, 2-ethylhexanoates, sulphonates) are used which are only rarely present in real analytical samples. Apart from Conostan®) oil standards, based on petrosulfonates, only a very limited range of elements are covered by commercial standards. As a result, many determinations by atomic spectroscopy methods realized in organic solvents, oils and other organic fluids are not properly calibrated. For example, Conostan® petrochemical based calibration standards are used for the calibration of the determination of many analytes in edible oils and oil standards based on cyclohexanebutyrates are used for the determination of wear metals in used lubricating oils where several analytes are present in the form of relatively large particles of metals or oxides, etc.